چکیده انگلیسی

Evaporative cooling by water spray is increasingly used as an efficient and environmentally-friendly approach to enhance thermal comfort in built environments. The complex two-phase flow in a water spray system is influenced by many factors such as continuous phase velocity, temperature and relative humidity patterns, droplet characteristics and continuous phase–droplet and droplet–droplet interactions. Computational Fluid Dynamics (CFD) can be a valuable tool for assessing the potential and performance of evaporative cooling by water spray systems in outdoor and indoor urban environments. This paper presents a systematic evaluation of the Lagrangian–Eulerian approach for evaporative cooling provided by the use of a water spray system with a hollow-cone nozzle configuration. The evaluation is based on grid-sensitivity analysis and validated using wind-tunnel measurements. This paper also presents a sensitivity analysis focused on the impact of the turbulence model for the continuous phase, the drag coefficient model, the number of particle streams for the discrete phase and the nozzle spray angle. The results show that CFD simulation of evaporation by the Lagrangian–Eulerian (3D steady RANS) approach, in spite of its limitations, can accurately predict the evaporation process, with local deviations from the wind-tunnel measurements within 10% for dry bulb temperature, 5% for wet bulb temperature and 7% for the specific enthalpy. The average deviations for all three variables are less than 3% in absolute values. The results of this paper are intended to support future CFD studies of evaporative cooling by water spray systems in outdoor and indoor urban environments.

مقدمه انگلیسی

As a result of climate change more buildings will be exposed to milder winters and hotter summers [1] and [2]. Research indicates that a major European heat wave, such as that of 2003, will occur more frequently in the future [3] and it could become a common event by 2040 [4]. Increased heat waves and heat stress are likely to cause increased illness and death as occurred in the hot summers of 2003 and 2006 [5]. These problems are aggravated by the urban heat island effect (UHI) [6] and [7]. The term urban heat island is used for urban areas which exhibit higher temperatures than their rural surroundings. Therefore, adaptation strategies such as evaporative cooling need to be evaluated and implemented to reduce heat stress in the outdoor and indoor urban environment.
Several research organizations and consortia have initiated projects regarding climate change adaptation in cities as the Intergovernmental Panel on Climate Change (IPCC) has expressed the importance of adaptation measures [8]. Climate Proof Cities (CPC) is one of these research consortia investigating the climate vulnerability of urban areas and the development of climate change adaptation measures [9]. The consortium consists of universities, research institutes, policy makers and city officials to perform both basic and applied science, the latter being an integrated and thorough analysis for several locations in the Netherlands.
Evaporative cooling by water spray is increasingly used as an efficient and environmentally-friendly approach to enhance thermal comfort in urban environments (outdoors and indoors) (e.g. Refs. [10] and [11]). In a water spray system, a cloud of very fine water droplets is produced using atomization nozzles. It enhances mixing and increases the contact surface area between the air stream and the water droplets resulting in a higher rate of evaporation yielding greater cooling of the ambient air.
For assessing the potential and performance of evaporative cooling by water spray systems in outdoor and indoor environments, different methods can be used: (i) full-scale measurements, (ii) wind-tunnel measurements, and (iii) numerical simulation with Computational Fluid Dynamics (CFD). Full-scale measurements offer the advantage that the real situation is studied and the full complexity of the problem is taken into account. However, full-scale measurements are usually only performed in a limited number of points in space. In addition, there is no or limited control over the boundary conditions. Reduced-scale wind-tunnel measurements allow a strong degree of control over the boundary conditions, however at the expense of – sometimes incompatible – similarity requirements. Furthermore, wind-tunnel measurements are usually also only performed in a limited set of points in space. CFD on the other hand provides whole-flow field data, i.e. data on the relevant parameters in all points of the computational domain [12], [13], [14] and [15]. Unlike wind-tunnel testing, CFD does not suffer from potentially incompatible similarity requirements because simulations can be conducted at full-scale. CFD simulations easily allow parametric studies to evaluate alternative design configurations, especially when the different configurations are all a priori embedded within the same computational domain and grid (see e.g. Ref. [14]). However, the accuracy and reliability of CFD are of concern, and verification and validation studies are imperative (e.g. Refs. [13], [16], [17], [18] and [19]). CFD is increasingly used to study a wide range of atmospheric and environmental processes (e.g. Refs. [13], [20], [21] and [22]). Examples include pedestrian wind comfort and wind safety around buildings [23], [24], [25] and [26], natural ventilation of buildings [12], [14], [27], [28], [29], [30], [31], [32], [33], [34] and [35], air pollutant dispersion [36], [37], [38] and [39], wind-driven rain [40] and convective heat transfer [41] and [42]. CFD has also been used on several occasions in the past to evaluate the performance of spray systems for different applications (e.g. Refs. [43], [44], [45], [46], [47] and [48]). In the vast majority of these studies the Lagrangian–Eulerian (LE) approach has been used in which the continuous phase (air in this study) is represented in an Eulerian reference frame while the discrete phase (water droplets in this study) is represented in a Lagrangian reference frame. The numerical implementation of this approach was introduced and applied by O'Rourke [49] and [50] and Dukowicz [51] for internal combustions engine applications. However, it has been developed and used for many other applications including evaporating spray systems. A comprehensive review of the LE method including its advantages over the Eulerian–Eulerian (EE) method, modelling issues and numerical implementation is provided by Subramaniam [52].
To the best of our knowledge, a detailed evaluation of the LE approach for predicting evaporative cooling has not yet been performed. This paper presents a systematic evaluation of the LE approach for predicting evaporative cooling provided by a water spray system with a hollow-cone nozzle configuration. The evaluation is based on grid-sensitivity analysis and on validation with wind-tunnel measurements by Sureshkumar et al. [53]. This paper also presents a sensitivity analysis focused on the impact of the turbulence model for the continuous phase and the number of particle streams for the discrete phase. In addition, the important impact of nozzle spray angle is demonstrated.
The results of this paper are intended to support future CFD studies of evaporative cooling by water spray systems in outside and inside urban environments.

نتیجه گیری انگلیسی

This paper presents a systematic evaluation of the Lagrangian–Eulerian approach for predicting evaporative cooling provided by a water spray system. This work was motivated by lack of knowledge on the accuracy and reliability of CFD for determining evaporative cooling provided by water spray systems. The evaluation is based on a grid-sensitivity analysis and on validation with wind-tunnel measurements by Sureshkumar et al. [53].
The present study showed that CFD simulation of evaporation by using the Lagrangian–Eulerian (3D steady RANS) approach, in spite of its limitations, can accurately predict the evaporation process with an acceptable accuracy. The local deviations from the wind-tunnel measurements are within 10% for dry bulb temperature, 5% for wet bulb temperature and 7% for the specific enthalpy. The average deviations for all three variables are less than 3% in absolute values. The impact of the turbulence model for the continuous phase, the number of particle streams for the discrete phase and the half-cone angle have also been investigated, and it was demonstrated that the selection of these parameters is very important for accurate and reliable results.